1. Field of the Invention
The present invention relates to the control of an electric generator set that includes an engine that mechanically drives an alternator, and in particular to regulating the voltage produced by the alternator.
2. Description of the Related Art
Electric generator sets (or “gensets”) are widely used to produce electric power. A genset typically includes an engine coupled to an alternator, which converts the rotational energy from the engine into electricity. The output terminal voltage of a genset is proportional to both the magnetic flux density within the alternator and the speed of the engine. The magnetic flux density is typically determined by controlling the field current, or excitation level, of the alternator, while the speed of the engine is typically determined by an engine governor.
Many conventional alternators are three-phase machines that output three different alternating voltages, each having a phase angle that is offset 120 degrees with respect to the phase angle of the other two voltages. It is typically desirable for a genset to produce an output voltage at a known, relatively constant level, since many loads are designed to receive power at a given voltage. In particular, the genset often is coupled to a power grid that is designed to maintain particular voltage levels. Because the output voltage of the alternator of a genset is determined in part by the excitation level of the alternator, it is important to be able to control this excitation level.
Control of the excitation level typically requires feedback information concerning the output voltage of the alternator. In one type of control, the output voltage for one electrical phase is fed to an analog voltage regulator that senses the output voltage and compares the sensed voltage to the desired voltage level. That comparison produces an error signal which is employed to adjust the excitation level of the alternator to bring the output voltage to the desired level. Such analog voltage regulators respond very quickly to voltage fluctuations resulting from transient load changes, however regulation based on an average voltage level can lead to inaccurate output voltages under other load conditions.
Digital voltage regulators, that use microcomputers, have been developed which utilize RMS voltage measurements, thereby avoiding the inaccuracies that occur with analog regulators. The digital voltage regulators provided good compensation for voltage variation that occurs under steady state electrical load conditions. However, reasonably priced digital voltage regulators react slower than analog regulators and may not respond adequately to voltage fluctuations from transient load conditions and their software is required to be customized for different alternator design.